calculating inrush to a capacitor bank

tjencks · 29th June 2011, 02:33 PM

So recently while working on an amp projected to build a couple Aussie Amplifier NVX-1200 amps I've run into a bit of an issue.

This amp is a monster and the Toroid is a 2KVA 80V beast. I do have an inrush limiter for the mains controlling power to the Toroid primaries however I've got a large bank of capacitors proceeding this. How large you say well 18 x 3900uF large.

I'm using an IXYS VBE60-06 rectifier bridge which is working just fine in one amp but have burned the other rectifier bridge up at switch on...it was pointed out to me that the inrush of the cap bank at switch on is likely an issue and that the amp which is working as a part that is just slightly better manufactured than the one that burned up thus avoiding the issue.

So my question really is how do I calculate current inrush at switch on worst case of course for my cap bank?

I have moved to a new bridge rectifier part that has a much higher surge rating than the old part 1200amps as opposed to 250amps.

any help appreciated.

ashok · 03:07 PM

Isn't a resistive surge supressor on the primary enough to control switch on surge in the capacitor also ? It's got to be in circuit long enough to avoid a big surge. Worst case condition is that the resistor might have to see almost a short condition at the instant you switch on. Wattage will have to be high enough to handle that surge for several milli seconds.

Under switch on condition the resistance in series will be the surge protection resistor + the primary coil resistance and the reflected secondary resistance. So the worst case surge current should be the mains voltage divided by the summed resistances. Right ?

Andy5112405 · 29th June 2011, 03:02 PM

250A - That should survive.

I've got 50A bridge with over 100000uF (yes 0.1F) and it survives with no inrush limitimg at all.

Are you sure you don't have a fault somewhere ?

I would reform the capacitors if I were you. Connect each one across the bridge with a large 10K or so resistor, let it build up to full Vcc and then leave it there for a few hours.

One of the caps may be short circuit.

Are they genuine caps ?

Did you connect them correctly ?

tjencks · 29th June 2011, 03:33 PM

Yeah they should all be genuine caps unless digikey sent me some bum parts...would really be a bummer to have to unsolder and test each one though.

I'm sure they are all in the correct orientation...but you could be right one of the caps could be bad...sigh if it's not one thing its another.

Is there a tool that can check check caps so next time I'll check each one before using it? I guess hooking up each cap and leaving it on for an hour or so seems a little slow to me...perhaps I'm just impatient ;-)

Thanks for the responses guys.

Best Regards,
Theo

Andy5112405 · 29th June 2011, 04:04 PM

When things go wrong you sometimes just have to be patient.

tjencks · 29th June 2011, 04:27 PM

Isn't that the truth...

I think I will bring things up very very slowly on my variac first and see how its looking..variac has a 5amp fuse and usually it starts to hum at around 8 or 9 volts if there is indeed a short.

For the cap test you mention how do I know I have a bad cap? I wouldn't want to find out by having one explode on me ;-)

DF96 · 29th June 2011, 04:39 PM

Calculating inrush is difficult. Better to let Spice or PSUD2 do the hard work for you.

Andy5112405 · 29th June 2011, 04:41 PM

With 10K in series its unlikely to explode.

Good caps will slowly build up to Vcc and stay there without drawing excessive current.

A bad cap just wont charge up. Even a shorted cap wont cause any grief as the current through a 10K resistor will be minimal. Use a 0.6W or 1W resistor.

Andy5112405 · 29th June 2011, 05:15 PM

You can REFORM a whole bank of capacitors together. If they are OK then NO PROBLEM. If one is faulty NO PROBLEM, you just wont know which one is faulty.

davada · 09:19 PM

Have a look at this sim.

I included the winding resistance in the source for the transformer I'm considering.

I found this white paper that addresses the issue. I have not read it, I only looked it over.
There are some useful calculation in the paper.

http://www.newark.com/pdfs/techartic...ntLimiters.pdf

You can use a NTC thermister or a large power resistor in series with the transformer primary.
Bypass the series element with a relay after the the first few cycles.

The relay can be activated by the supply Voltage if you use say a 48Vdc with a series resistor on the coil. That simple.

David.

29th June 2011, 02:33 PM	#1
tjencks diyAudio Member Join Date: Feb 2009	calculating inrush to a capacitor bank So recently while working on an amp projected to build a couple Aussie Amplifier NVX-1200 amps I've run into a bit of an issue. This amp is a monster and the Toroid is a 2KVA 80V beast. I do have an inrush limiter for the mains controlling power to the Toroid primaries however I've got a large bank of capacitors proceeding this. How large you say well 18 x 3900uF large. I'm using an IXYS VBE60-06 rectifier bridge which is working just fine in one amp but have burned the other rectifier bridge up at switch on...it was pointed out to me that the inrush of the cap bank at switch on is likely an issue and that the amp which is working as a part that is just slightly better manufactured than the one that burned up thus avoiding the issue. So my question really is how do I calculate current inrush at switch on worst case of course for my cap bank? I have moved to a new bridge rectifier part that has a much higher surge rating than the old part 1200amps as opposed to 250amps. any help appreciated.

29th June 2011, 02:58 PM	#2
ashok diyAudio Member Join Date: Jun 2002 Location: 3RS	Isn't a resistive surge supressor on the primary enough to control switch on surge in the capacitor also ? It's got to be in circuit long enough to avoid a big surge. Worst case condition is that the resistor might have to see almost a short condition at the instant you switch on. Wattage will have to be high enough to handle that surge for several milli seconds. Under switch on condition the resistance in series will be the surge protection resistor + the primary coil resistance and the reflected secondary resistance. So the worst case surge current should be the mains voltage divided by the summed resistances. Right ? __________________ AM Last edited by ashok; 29th June 2011 at 03:07 PM.

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29th June 2011, 03:02 PM	#3
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	250A - That should survive. I've got 50A bridge with over 100000uF (yes 0.1F) and it survives with no inrush limitimg at all. Are you sure you don't have a fault somewhere ? I would reform the capacitors if I were you. Connect each one across the bridge with a large 10K or so resistor, let it build up to full Vcc and then leave it there for a few hours. One of the caps may be short circuit. Are they genuine caps ? Did you connect them correctly ?

29th June 2011, 03:33 PM	#4
tjencks diyAudio Member Join Date: Feb 2009	Yeah they should all be genuine caps unless digikey sent me some bum parts...would really be a bummer to have to unsolder and test each one though. I'm sure they are all in the correct orientation...but you could be right one of the caps could be bad...sigh if it's not one thing its another. Is there a tool that can check check caps so next time I'll check each one before using it? I guess hooking up each cap and leaving it on for an hour or so seems a little slow to me...perhaps I'm just impatient ;-) Thanks for the responses guys. Best Regards, Theo

29th June 2011, 04:04 PM	#5
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	When things go wrong you sometimes just have to be patient.

29th June 2011, 04:27 PM	#6
tjencks diyAudio Member Join Date: Feb 2009	Isn't that the truth... I think I will bring things up very very slowly on my variac first and see how its looking..variac has a 5amp fuse and usually it starts to hum at around 8 or 9 volts if there is indeed a short. For the cap test you mention how do I know I have a bad cap? I wouldn't want to find out by having one explode on me ;-)

29th June 2011, 04:39 PM	#7
DF96 diyAudio Member Join Date: May 2007	Calculating inrush is difficult. Better to let Spice or PSUD2 do the hard work for you.

29th June 2011, 04:41 PM	#8
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	With 10K in series its unlikely to explode. Good caps will slowly build up to Vcc and stay there without drawing excessive current. A bad cap just wont charge up. Even a shorted cap wont cause any grief as the current through a 10K resistor will be minimal. Use a 0.6W or 1W resistor.

29th June 2011, 05:15 PM	#9
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	You can REFORM a whole bank of capacitors together. If they are OK then NO PROBLEM. If one is faulty NO PROBLEM, you just wont know which one is faulty.

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